A glass is formed on cooling a liquid (if crystallization can be avoided), faster cooling giving more disordered, higher-energy states1. The difference between the highest and lowest energies attainable in the glass at a given temperature is remarkable, nearly as large as the enthalpy of melting2. After casting, annealing allows relaxation or ageing of glassy states to lower energies, while the opposite process, rejuvenation, can be induced by reheating and faster cooling3, and, most commonly, by plastic deformation1, 4. Deformation broadens the range of interatomic distances in a metallic glass, a clear sign of disordering opposite to the effects of relaxation5. Such studies have links with the interest, for crystalline metals, in tailoring properties by control of defect structures at a fixed composition6.
Viscous flow of metallic glasses near their glass-transition temperature Tg is homogeneous, but their plastic flow at room temperature (RT) shows an instability in which shear is sharply localized in bands that may be as thin as 10–20 nm7, 8. This inhomogeneous deformation leads to essentially zero tensile ductility, and is the main impediment to wider structural use of metallic glasses. Rejuvenation reduces the initial yield stress and, it is speculated, could ultimately eliminate the undesirable shear-banding1. Extreme rejuvenation is also of interest in exploring the limits of glass formation and stability.
The usual inhomogeneous nature of plastic flow in metallic glasses itself limits the degree of rejuvenation that can be achieved, because the regions of significant strain occupy only a small volume fraction of the specimen. Studies of a single shear band4, 9 show that the effects of shear can extend into the glassy matrix by some tens of micrometres, far beyond the thickness of the band itself. However, the effects (softening and increased enthalpy) are sharply peaked at the band centre, and the volume fraction of the glass that is strongly affected is small. Rejuvenation in the bands themselves is inefficient as the structural effects of deformation are likely to saturate for shear strains greater than one. Also, rejuvenated states may not be fully retained because of relaxation facilitated by local heating8.
To achieve significant flow and rejuvenation throughout a deformed metallic glass, it would be helpful for shear-banding to be suppressed. Here we show that under constraint a metallic glass can be compressed to large strains (up to 40%) in a regime of presumed homogeneous flow. The constraint is achieved in notched specimens. Previous work has shown that plastic flow in notched specimens under tension can lead to relaxation rather than rejuvenation10, and we analyse the distinction between these cases. Under compression, significant volumes of the metallic glass can attain degrees of rejuvenation previously associated only with the central plane of shear bands. The states attained can have energies so high that they would be characteristic of a glassy state obtained by quenching at ~1010 K s–1.